The invention pertains to the field of carrying out keyless access authorization control for checking, for example, whether a person is authorized to perform a certain action, such as opening a vehicle door. In particular, the invention pertains to a method for carrying out keyless access authorization control by means of wireless communication between a transceiver unit corresponding to a base station and a mobile identification transmitter (ID transmitter) in order to check the access authorization of the person carrying the ID transmitter. The method includes: transmitting a code signal from the transceiver unit of the base station, receiving this code signal by the ID transmitter, carrying out an action that can be detected by the transceiver unit and that is executed by the ID transmitter at the receipt of the code signal as a reply signal of the ID transmitter, and detecting the action of the ID transmitter and determining the distance between the base station and the ID transmitter.
The invention also pertains to a keyless access authorization control device with a base station featuring a transceiver unit as well as a control and evaluation element and a mobile identification transmitter (ID transmitter) featuring a transceiver unit for checking the access authorization of the person carrying the ID transmitter. This base station can calculate the distance from the base station to the ID transmitter.
Keyless locking systems are used in numerous applications, for example, in vehicles to increase user convenience. Conventionally, infrared and radio systems are used as remote-control systems wherein the authorized user activates the ID transmitter in order to transmit a signal to the base station, for example, to a receiver unit in a vehicle for opening the vehicle. To further increase user convenience, passive access authorization control systems can be used as the user passes between vehicles, so that an authorized user carrying a valid ID transmitter can open his vehicle without having to actively turn on the ID transmitter. Such a keyless access authorization control device is described, for example, in DE 43 29 697 C2. In the method disclosed in this document, a code signal is transmitted from a transceiver unit in a vehicle, and this signal is received by the ID transmitter when the transmitter is within the reception area of this signal. Then, a reply signal is sent back from the ID transmitter due to the action triggered by the receipt of the code signal. The validity of the reply signal is checked after it is received by the receiver unit of the vehicle. The receipt of a valid reply signal triggers the desired action in the vehicle, namely unlocking the vehicle doors. This passive system is problematic in that the transmission path between the base station and the ID transmitter can be extended by simple means that are unauthorized and unnoticeable to the authorized user. In this way, an unauthorized person can establish radio connection between the transceiver unit of the vehicle and the ID transmitter authorized for opening, even if the latter is not within the reception area of the request signal. Thus, the unauthorized person can gain unknown entry to the vehicle.
The object of DE 196 32 025 A1 addresses this problem. In the keyless access authorization control device described in this document, detection of the propagation time relative to the transmitted request signal and the received reply signal is carried out, in which the corresponding time span required by the transmitted code signal to be transmitted from a base station and then received as a reply signal is calculated. If the transmission path has been lengthened, then the time interval from the transmission of the code signal to the receipt of the reply signal is naturally greater than when the ID transmitter is definitely in the direct vehicle proximity and the corresponding return radio path is short. If the calculated signal propagation time exceeds a predefined value, the access authorization control method is stopped in order to prevent unauthorized entry.
In order to carry out the access authorization control method described in this document, it is necessary to detect the signal propagation time with high accuracy. Communication between the transceiver unit and the mobile ID transmitter is typically within a very limited area around the base station: for vehicles, an area between 5 and 10 m e.g. The propagation time of an ordinary transmitted and received signal within this distance is between 16.5 ns and 33 ns. The detection of such short time spans, in particular with the desired resolution, is only possible with significant expense, an expense that is not justifiable for numerous applications, for example, for a vehicle.
Starting with the previously discussed prior art, the task of the invention is based on proposing such a method for carrying out keyless access authorization control that not only features sufficient security relative to the possibility of detecting a manipulated path extension, but that can also be realized with the use of simple means.
The task of the invention is also based on proposing such a keyless access authorization control device that can detect a (manipulated) path extension with the use of justifiable means.
The task related to the method is solved according to the invention in that the distance determination is done based on a relative determination, in which the difference of a distance-dependent signal characteristic correlated with the signal propagation time between a reference code signal monitored in the base station and the reply signal received by the transceiver is evaluated.
The knowledge of the claimed invention is based on the fact that it is possible to determine distance with sufficient resolution over indirect paths by detecting a signal value correlated with the propagation time of a signal using simple means. In contrast to the previously known method, this distance determination is done by means of a relative determination, in which a reference code signal is monitored in the base station with reference to the transmitted signal characteristic and the signal characteristic of this reference code signal is compared with the signal characteristic of the reply signal returned by the ID transmitter and received by the transceiver unit. The distance-dependent signal characteristic to be compared with the reference code signal gives information about the return path of the code signal and reply signal when compared with the corresponding reference code signal. In this way, the reference code signal, for example, the transmitted code signal, can be monitored during the transmission period of the code signal by the base station. The detection by the base station of the actions executed by the ID transmitter is limited by the measurement time distance. These actions can be, for example, the return transmission of the code signal received by the ID transmitter. The actions executed by the ID transmitter can also be other actions that can be detected by the base station, such as the shutdown of a transmitter and thus the detection by the base station of the time when such a transmitter is turned off by the ID transmitter. The use of hardware to carry out such a relative distance determination is very minimal in comparison with that required to carry out an absolute propagation time measurement. Depending on the method used, numerous evaluation steps can also be eliminated during data processing.
Starting with the background of this embodiment and the description of the invention, either the delay times by the circuit elements used are much shorter relative to the actual signal propagation time or they are known, or they are calculated and taken into consideration in the evaluation.
To carry out such a relative distance determination, different signal characteristics can be used. For example, counting the cycles of the carrier wave of the transmitted code signal over a time period until, for example, a reply signal has been received by the transceiver unit can be done in the base station. The number of cycles counted in the base station over the time until the detection of the performed action, for example, the return transmission of the received code signal, multiplied by the period of the carrier wave gives the propagation time of the signal from the base station to the ID transmitter and back. The distance between the base station and the ID transmitter is then given by half the signal propagation time multiplied by the speed of light. Thus, the propagation time cannot be detected in continuous increments, but rather in multiplies of the period. Using a carrier frequency of 100 MHz, for example, the resulting distance determination between the base station and the ID transmitter has a resolving power of approximately 1.5 m. Using a carrier wave in an ISM band, with an approximate frequency of 434 MHz, a resolving power of approximately 0.35 m can be achieved due to the correspondingly smaller period.
To realize the method according to the invention using this configuration, electronic components that are otherwise used to perform authorization requests can be used. Additional requirements are merely the use of a counter that can be designed, for example, out of logic gates or flip-flops. Thus, the method according to the invention can be realized without great additional expense.
Expediently, the wireless communication is performed in a frequency distance within an ISM band. The resolving power of the distance determination during use of such a frequency is considerably higher than that required for the use of such a method for keyless access authorization control for a vehicle. The resolving power for using such a method for vehicles is on the order of 5-10 m. Due to the minimal resolving power required in such an application, compared with the possible resolving powers, the requirements on the counter can be lessened such that not every cycle, but rather every xth, or approximately every 2xth cycle, is counted. Such selective cycle counting can be realized through the use of a frequency divider or a frequency mixer, so that for a dividing ratio of 1:16, only every 16th cycle is counted. Using a frequency of 434 MHz and a dividing ratio of 1:16 results in a counting frequency of 27.125 MHz and consequently a resolving power of approximately 5.5 m; a resolving power that is sufficient for using the method for vehicles, relative to distance determination.
In a refinement of this embodiment, the command TRANSMITTER ID TRANSMITTER ON is transmitted by the transceiver in a first step for distance determination. Based on this command, a transmitter is turned on in the ID transmitter in order to transmit at a first frequency. After a predetermined length of time that is sufficient for the transmitter of the ID transmitter to have transmitted, a code signal is transmitted at a second frequency by the transceiver unit of the base station. This code signal is the command TRANSMITTER ID TRANSMITTER OFF. Simultaneous with the transmission of the command, the cycle count of the carrier wave of this transmission signal begins. Beforehand, the receiver channel of the transceiver unit is switched to the first frequency so that the signal transmitted by the ID transmitter can be received. The cycle count is stopped when the transmission signal of the ID transmitter can no longer be detected by the transceiver unit of the base station. Thus, the cycle count is performed for a length of time necessary for the propagation time of the code signal from the base station to the ID transmitter and back. The action signal of the ID transmitter detected by the base station is the reaction of the ID transmitter to the transmitted code signal.
In another configuration of this embodiment, the code signal transmitted by the base station is returned by the ID transmitter in a mirror-like fashion. In this embodiment, the cycle count is stopped when the code signal as the reply signal of the ID transmitter has been completely received again by the transceiver unit of the base station. In another embodiment, in order to be able to compensate for possible data loss during data transmission, two cycle counters are assigned in the base station, wherein a first cycle count is coupled to the transmission of the code signal and another cycle count is coupled to the receipt of the code signal reflected by the ID transmitter. The cycle count of the second counter coupled to the receipt of the code signal is stopped when the counter associated with the transmission of the code signal reaches a count that corresponds to twice the number of cycles corresponding to the code signal. The greater the distance between the base station and the ID transmitter, and correspondingly, the longer the signal propagation time, the larger is the difference between the calculated counter status associated with the receipt of the code signal and the number of cycles corresponding to the code signal. For the last two examples, it is expedient to transmit a predetermined number of cycles of a carrier wave.
Another embodiment of the invention takes advantage of the fact that the phase of a wave transmitted over a radio path is shifted depending on the return radio path relative to the originally transmitted phase. In this way, both the phase of the frequency and also the amplitude variation (envelope) can be used individually or together in order to compare the phase of the transmitted request signal with the corresponding reply signal returned by the ID transmitter. In this way, the reply signal is transmitted from the ID transmitter to the transceiver of the base station using a carrier wave. The carrier wave can be a wave within the reply signal, for example, the request signal itself, or it can be transmitted to the ID transmitter by modulating a carrier wave for transmitting the request signal. If the ID transmitter is located within the predetermined receive area of the transmitted request signal, which is less than 3 m for the application of the method to the automotive field, the return path (transceiver unit-ID transmitter-transceiver unit) is short. The phase shift between the demodulated reply signal and the originally transmitted request signal is so small that it can be produced within a predetermined tolerance distance of phase-locked signals (request signal and reply signal). For a manipulated path extension, the phase between request signal and reply signal is constantly shifted, so that this is outside the tolerance distance of the operation defined as phase-locked. The probability of the received and demodulated reply signal having a phase corresponding to the respective request signal is purely random and thus it is extremely small. Consequently, the desired action, like the opening of a vehicle, is only performed when the ID transmitter is at the predetermined distance to the base station.
To prevent undesired feedback, the request signal is expediently transmitted at a different frequency than the reply signal. In an especially simple configuration, the request signal is transmitted at the frequency for modulating the reply signal. This can be realized so that, for example, the request signal is transmitted on a low-frequency channel and this request signal received by the ID transmitter is used to directly modulate a carrier wave on a high-frequency channel. For example, a request signal can be a wake signal transmitted on an LF channel for switching the ID transmitter from quiet or sleep mode to active mode.
To further increase access security, the oscillator for transmitting the request signal is operated in a free-running manner, so that it produces known deviations in frequency. In addition, the transmission frequency can be changed according to a predetermined variation pattern so that the chance of realizing an unauthorized and yet phase-locked path extension is even further reduced.
In another configuration of this embodiment, a modulated carrier wave with a modulation scheme of one or more modulation variables is transmitted as a request signal by the transceiver unit. In this way, either a modulation scheme of different frequencies or different amplitudes or even a scheme of these two values is used. After receipt of the reply signal modulated in this way, filtering relative to the original modulation scheme components, for example, the individual frequency components, is performed after demodulation of the reply signal. The subsequent step of the phase comparison of the originally transmitted request signal with the received reply signal is then performed with reference to the individual components forming the modulation scheme, for example, the frequency components. The use, for example, of a modulation scheme of different frequencies for modulating the request signal increases the operation security of the claimed method, in particular, to the effect that a chance phase equalization for an unauthorized path extension can be recognized due to the possibility of an absolute distance measurement of the return radio path. For the use of a frequency scheme, a base frequency is used and mixed expediently with a defined number of other frequencies that are each components of the base frequency divided by two. This can be realized in a simple way with a frequency divider. In this way, the value of the base frequency defines the resolution of the distance measurement and the number of divisions of the maximum detectable distance.
A phase comparison between the originally transmitted request signal and the received reply signal can be realized, for example, through the use of a phase comparator.
In another embodiment of the invention, a modulated carrier wave transmitted over a radio path can be evaluated depending on the length of the return radio path with reference to a change or replacement of the function value of the used modulation variables compared with simultaneous function values of the originally transmitted signal. This method takes advantage of the time-dependent shift of the modulation curve of the reply signal relative to the identical modulation curve of the request signal due to the distance between the base station and ID transmitter. According to the claimed method, a function value of the modulation variables of the received reply signal at a certain time is compared with the simultaneous function value of the modulation variables of the originally transmitted request signal. Such a comparison is expediently performed by the step of a difference formation of absolute values or also squares of the function values of the modulation variables at a predetermined time. This function value comparison can be performed at quasi-arbitrary positions of the modulation function and thus, continuously.
To increase access security, the frequency of the carrier wave for the request signal is different from that of the corresponding reply signal. It is expedient to transmit the request and/or reply signal on respective paths, on which there occurs an encrypted data dialog between the base station and the ID transmitter, if necessary.
In one configuration of this method, the carrier wave is transmitted frequency modulated, for easier evaluation, preferably linearly frequency-modulated. Correspondingly, the base station has a means for transmitting such a carrier wave as a request signal, as well as an FM demodulator that performs a demodulation of the received reply signal. The mobile ID transmitter has an FM demodulator for demodulating the received request signal. The output of the demodulator is expediently connected to an input of the transceiver for data communication, so that the data signal transmitted from the ID transmitter simultaneously represents or contains the reply signal to the transmitted request signal due to its modulation.
The comparison of a function value of the modulation variables, for example, the modulation frequency of the received reply signal, with the simultaneous function value of the modulation frequency results in a difference frequency. The measurement of this difference frequency is a measure of the length of the return radio path (base station-ID transmitter-base station). Authorization control is expediently performed such that a threshold for the difference frequency is preset, wherein when this threshold is exceeded, the authorization is denied.
Alternative to the use of a frequency-modulated carrier wave, an amplitude-modulated carrier wave or also a frequency and amplitude modulated carrier wave can be used to characterize the request and reply signal.